DSL methods (DSL=digital subscriber line) and in particular asymmetric DSL methods (ADSL=asymmetric digital subscriber line) are increasingly used for quick access to the Internet. In this situation, ADSL differs from symmetric DSL methods (SDSL=symmetric digital subscriber line) essentially in that in the case of ADSL different transfer rates are provided for uplink (transfer from subscriber to the internet) and downlink (transfer from the internet to the subscriber). In this situation, the downlink transfer rate is regularly significantly (by a factor of 4 or 6, for example) higher than the uplink transfer rate since typically more data is transferred per unit of time over the downlink than over the uplink. In the case of SDSL, on the other hand, the transfer rates for downlink and uplink are the same.
In the following the term “DSL” is used as a generic term both for “ADSL” and also for “SDSL” since the actual transfer rates and the ratio of the transfer rates between uplink and downlink are of no relevance to the present invention.
A data terminal device of the subscriber, for example a PC (PC=personal computer) or a set-top box, is connected via a conventional copper twin wire to an access multiplexer (DSLAM) by means of an integrated or external modem to a DSLAM (DSLAM=DSL access multiplexer). The DSLAM is located in an access node of the network provider, which provides the connection for the subscriber. Typically, a plurality of subscribers is connected via corresponding interface modules of a DSLAM to the DSLAM.
In conventional DSL access networks, the DSLAM is linked to downstream network elements by means of an ATM network (ATM=asynchronous transfer mode).
The disadvantage in this situation is the fact that ATM networks are expensive and complicated both in their construction and in their operation since on the one hand the components of an ATM network (switches for example) are expensive in comparison with components which support other transfer technologies and on the other hand each connection in an ATM network needs to administered separately.
In order to avoid this disadvantage, “DSL: The Corporate Connection”, T. Jessup, Data Communications Vol. 27, No. 2 (February 1998), pp. 103-108, McGraw Hill, New York, proposes for example that the DSL access lines be connected using ATM networks, Ethernets, T1 connections, serial lines or frame relays.
If the subscriber-side termination of the connection is implemented cost-effectively using Ethernet, there is however a danger that as a result of the self-configuration capabilities of the Ethernet bridge EB required in conjunction with the access multiplexer DSLAM an attack on the DSLAM using a large number of falsified MAC addresses (MAC=media access control) causes the routing tables of the DSLAM to overflow. In addition, in the case of unfiltered Ethernet traffic, unauthorized connections between the data terminal devices D1, D2, D3 are possible, for example.